(1) Field of the Invention
The present invention relates to intervertebral disks prostheses.
Degenerative processes due to a trauma, diseases or aging often lead to the replacement of an intervertebral disk.
These degenerative processes can involve a change in the natural space between two vertebrae. The contraction of this natural space can cause a pressure which acts on certain nerves, and as a result pain can occur.
A disk prosthesis can be used to maintain the natural space between two vertebrae.
It must also allow the vertebrae to move in relation to each other following a natural movement. In particular it must allow an axial rotation movement which corresponds in part to the rotational movement of the trunk or neck of the human body in the cervical region, an anteroposterior movement which corresponds to a movement bending or extending the upper part of the body or head in the cervical region, and a lateral movement which corresponds to a tilting of the upper part of the body or the neck in the cervical region.
The upper and lower plateaux of a disk prosthesis bear on the vertebrae and over time become fixed thereto as a result of surface treatments and the roughnesses with which they are generally provided.
(2) Description of Related Art
EP-A1-1 344 508 describes an intervertebral disk prosthesis comprising an upper plateau, a lower plateau having a bearing surface, an intermediate part comprising a lower surface and an upper surface having a spherical part, the upper plateau having an impression complementing the spherical part and coming into contact with the spherical part so as to define a ball-socket joint between the upper plateau and the intermediate part, the lower surface of the intermediate part coming into contact with the bearing surface.
The intermediate part can move forwards and backwards on the bearing surface of the lower plateau.
However, this type of prosthesis does not provide the desired comfort and robustness, in particular during violent impacts.
WO2004/016217 describes intervertebral disks prostheses. A wide variety of configurations is proposed. In one of these, there are provided a lower plateau, an upper plateau and an intermediate part comprising an upper convex surface to cooperate with a cavity created in the upper plateau. The intermediate part comprises long narrow horizontal notches.
However, in order to give an adequate shock absorption, this type of prosthesis would necessitate very deep notches creating fatigue rupture cracks which would considerably weaken the prosthesis and in particular would not provide the desired robustness, particularly during violent impacts or rotation.